The present invention pertains to heat transfer devices and, more particularly, to heat pipe air preheaters used in large-scale industrial processes, such as steam generation for electric generating plants.
Heat pipe air preheaters essentially consist of a bundle of self-contained heat pipes. Each heat pipe is partially filled with a working fluid, most commonly water or hydrocarbon, and sealed. Heat from, for example, flue gas evaporates the working fluid collected in the lower or evaporator end of the slightly inclined pipe (generally 5 to 15 degrees from the horizontal) and the vapor flows to the upper or condenser end, where it gives up heat to the, for example, incoming combustion air.
Condensed fluid returns by gravity to the evaporator end. The process continues indefinitely as long as there exists a temperature difference between the, in this example, flue gas and combustion air. The capacity of the individual heat pipe depends upon several factors, including its inclination angle and the temperature differential between its ends, increasing both as the inclination angle and the temperature differential increase.
In a typical design, heat pipes are attached at their midpoints to a divider plate which both supports the pipes and provides a barrier between the counterflowing flue gas and combustion air. The individual heat pipes are arranged in parallel, superposed rows. On one side of the divider plate, flue gas flows through the rows in one direction, transferring heat to the evaporator ends of the heat pipes, while on the other side of the plate combustion air flows through the rows, most commonly in the opposite direction, absorbing heat from the condenser ends of the pipes. Thus, the temperature differential of the heat pipes in rows at one end of the preheater differs from that of pipes in rows at the opposite end of the preheater. This, in turn, results in the heat pipes of at least some of the rows operating at less than optimal capacity.
Typically, also, heat pipe air preheaters are subject to severe space constraints. The problems thus imposed are compounded by the length of the heat pipes, which is commonly 40 feet or more. Thus, increasing the inclination angle by just one degree results in an increase in the overall height of the air preheater of more than 8 inches.
It is, therefore, a primary object of the present invention to provide a heat pipe air preheater wherein all of the individual heat pipes are operating at optimal capacity.
It is a further object to provide such an air preheater which has a minimal height.
The foregoing and other objects and advantages are achieved by a heat pipe air preheater wherein a multiplicity of heat pipes are arranged in a plurality of superposed, planar rows inclined relative to the horizontal, the rows of heat pipes including a first group of rows inclined at a first inclination angle and a second group inclined at a second inclination angle.
More particularly, the air preheater includes means for removing collections of soot or other particulate matter from the evaporator ends of the heat pipes, and the rows of the first group are disposed on one side of these means while the rows of the second group are disposed on the other side, the rows of the two groups converging in the direction of the heat pipe condenser ends.